Copyright 2006 The National Association of Certified Home Inspectors Roof Framing Roof types Gable – Most common, built with “common” rafters Hip – Provides overhang on all four sides Gambrel – Provides more space on second floor Mansard – Combination of Hip and Gambrel Shed- Frequently used to attach one structure to another Copyright 2006 The National Association of Certified Home Inspectors

Copyright 2006 The National Association of Certified Home Inspectors Roof Framing Definitions Common rafter – Runs from top plate to ridgeboard of a gable roof Hip rafter – Runs from corner of top plates to ridgeboard on a hip roof Jack rafter – any rafter which does not run the full length from plate to ridge ( e.g. – Hip jack, Valley jack) Copyright 2006 The National Association of Certified Home Inspectors

Copyright 2006 The National Association of Certified Home Inspectors Roof Framing Roof Geometry Definitions Span – Measurement from outside of wall to outside of opposite wall Run – One half of span (for symmetric roofs) Rise – The total vertical distance that the roof projects above the top plate Slope – The rise divided by the run, always given in terms of 12” of run (e.g. 3 on 12 written 3/12) Pitch – The rise over the span Copyright 2006 The National Association of Certified Home Inspectors

Copyright 2006 The National Association of Certified Home Inspectors Roof Framing Overhang – The section of the rafter extending past the edge of the wall Projection – The horizontal distance that the overhang covers Rafter tail cuts – Cuts made to form the overhang Birdsmouth – Cuts made to sit on the top plate Ridge cut – Cut made to attach to the ridgeboard Copyright 2006 The National Association of Certified Home Inspectors

Copyright 2006 The National Association of Certified Home Inspectors Roof Framing Rafters vs. Trusses Rafters used frequently for remodeling, for cathedral ceilings, for shed roof additions, for full 2nd floor storage, and spans up to 24’ Trusses used in most new construction, for spans 24’-60’, and most commonly for lower sloped roofs Copyright 2006 The National Association of Certified Home Inspectors

Copyright 2006 The National Association of Certified Home Inspectors Roof Framing Sizing Rafters – Rafter size (like span tables for floor joists) depends on spacing, species, load, and span. Sizing of rafters typically based on snow load in Northeast. The specific loads come from the International building code Copyright 2006 The National Association of Certified Home Inspectors

Copyright 2006 The National Association of Certified Home Inspectors Roof Framing Dormers – Most dormers are either shed or gable dormers. They are framed with common rafters. Copyright 2006 The National Association of Certified Home Inspectors

Roof Trusses

Definition Load Transferred Load not Transferred Load Transferred

Truss Types Trusses are categorised into 3 groups depending on the shape of the top chord Triangular Roof Trusses Crescent roof Trusses Other Types

Triangular Roof Trusses Simple Triangular geometric shape Web Bracing Straight Top Chord

Triangular Roof Trusses

Crescent Roof Trusses Top Chord is manufactured with a curved top chord The Harbour bridge is a good example

Other Types Top Chords may be parallel – such as floor joist trusses Or they may be nearly parallel – such as bridges

Roof Truss Members

Roof Truss Stress Types

Parallel Chord Trusses Top Chord & Bottom Chord are parallel Used as Rafters Advantages Lighter Larger Spans Allow for easy access for services Disadvantages Cannot be site modified

Parallel Chord Trusses Top Chord & Bottom Chord are parallel Used as Rafters Advantages Lighter Larger Spans Allow for easy access for services Disadvantages Cannot be site modified Generally Deeper

Truss Manufacture Designed by Structural Engineer No Site Modification or repair without engineer supervision Trusses manufactured in controlled factory conditions to ensure design is strictly followed

Camber Trusses are manufactured with camber in the bottom chord To allow for calculated deflection while dead loads such as Roof Covering & Ceiling Linings Bottom chord should not be supported between supports, unless specifically designed

Transfer of Loads Click to show load flow on correctly installed trusses Tensile Load to Counteract Compressive Load Compression Load Internal Wall Min 12 Clear No load in this Area

Transfer of Loads Click to show load flow on incorrectly installed trusses Bottom Chord is not designed to take horizontal load and will fail Bottom Chord bearing on Internal Wall Load transmits Horizontally to wall.

Complex Roofs Trusses can be made to suit just about any roof shape

Complex Roofs Trusses can be made to suit just about any roof shape

Truss Layout

Trussed Hip Roofs Jack End Truss

Hip Truss

Truncated Girder Truss Placed at position determined by Engineer Will take load of Jack Truss, Hip Truss & Creeper Truss Designed to take more load than other trusses

Truncated Truss Placed between girder truss and gathering point Important to be placed at specified spacing at they will increase in height to match the hip end Made to similar specification to standard truss as they take no extra load